1. Field of the Invention
The present invention generally relates to an inkjet recording apparatus that forms an image by ejecting an ink droplet onto a recording medium and, more particularly, to a recording-head driving circuit that outputs a driving signal to a piezoelectric element for use in ejecting the ink droplet.
2. Description of the Related Art
An inkjet recording apparatus is known as one of image forming apparatuses including printing machines, facsimile machines, copier machines, and multifunction peripherals having two or more functions of these machines. The inkjet recording apparatus generally forms a desired image by ejecting ink droplets onto a recording medium, such as paper or transparency, from an inkjet recording head.
Such an inkjet recording head typically employs a method that uses a piezoelectric element as a pressure generator for pressurizing ink in an ink channel, and ejects an ink droplet by causing the piezoelectric element to micro-vibrate a diaphragm, which is a wall of the ink channel, thereby changing internal volume of the ink channel.
An overcurrent anomaly caused by a short-circuited load or an anomaly related to power supply voltage, such as an over voltage or a low voltage, can occur in an inkjet recording apparatus. There are conventionally known techniques for such an anomaly. One of the techniques protects circuitry by providing a power-supply control circuit in the inkjet recording apparatus and stopping power supply to the inkjet recording head and to a driving circuit at occurrence of an anomaly. Another technique prevents ejection of an unnecessary ink droplet, which can occur due to residual charges at occurrence of an anomaly, by applying a voltage that is in anti-phase with a driving voltage supplied to the piezoelectric element.
For instance, Japanese Patent Application Laid-open No. 2011-037196 discloses a technique (first conventional technique) for protecting a driving circuit from an anomalous short circuit between various connecting lines that can occur in an unexpected manner. According to this technique, when an anomalous short circuit is detected, a control circuit stops power-supply output from a power-supply generating circuit and the like, thereby stopping supplying power source to load of an inkjet recording head. Simultaneously, the control circuit stops supplying power source to a circuit(s) (more specifically, circuit components connected to a load power-supply line), to which the power source is supplied from the power-supply generating circuit, among the driving circuit.
According to this first conventional technique, supply of power source to a drive-voltage output circuit (specifically, an isolator driver IC or the like) can be stopped, and therefore supply of the drive voltage to the inkjet recording head can be stopped.
Japanese Patent No. 3252628 discloses a second conventional technique. According to this technique, a control circuit applies a refresh pulse voltage that is in anti-phase with an applied pulse voltage (drive voltage) in order to eliminate residual charges between a diaphragm and an electrode, thereby preventing an unnecessary ink droplet from being ejected by an electric field produced by the residual charges. In the second conventional technique, during normal driving, electrodes are formed respectively both sides of the diaphragm that has a function as a capacitor, and the electrodes is applied with voltages to generate mechanical vibrations, thereby the ejection is performed.
According to the second conventional technique, constants of an external resistor and a capacitor, which is made up of the diaphragm and the electrode, are set to desired values, thereby determining a time constant over which the residual charges are to be discharged. By causing a trailing edge of the refresh pulse voltage to be sloped, ejection of an unnecessary ink droplet can be effectively prevented.
However, the control method according to the first conventional technique that stops supplying the power source to the inkjet recording head and to the driving circuit when printing operation is stopped is disadvantageous in that a rise rate of the voltage applied to the piezoelectric element is uncontrollable. This is because the voltage applied to the piezoelectric element is placed in a transient state when the power supply is stopped, and the applied voltage depends on a load capacitance connected to the power source.
An inkjet recording head ejects an ink droplet by applying a voltage onto a piezoelectric element to deform the piezoelectric element; accordingly, the ink droplet ejection depends on a deformation amount and a deformation rate of the piezoelectric element. In particular, when the load capacitance connected to the power source is small, the applied voltage falls rapidly, and therefore the piezoelectric element deforms rapidly, resulting in ejection of an unnecessary ink droplet. Furthermore, the rapid deformation leads to early degradation of the piezoelectric element, which is also disadvantageous. These problems are specifically described later.
The second conventional technique that applies to the piezoelectric element the voltage that is in anti-phase with the drive voltage is effective in preventing ejection of an unnecessary ink droplet caused by residual charges during normal driving. However, this technique is incapable of solve the problem of ejection of an unnecessary ink droplet that occurs at occurrence of an anomaly because a printing pulse falls earlier than the refresh pulse voltage is applied.
There is a need for an inkjet recording apparatus capable of, at occurrence of an anomaly in the inkjet recording apparatus, protecting a reference-voltage generating circuit that generates a reference voltage for use in driving a piezoelectric element, and simultaneously preventing ejection of an unnecessary ink droplet from an inkjet recording head and early degradation of the piezoelectric element.